1. Field of the Invention
The present invention relates to electron microscopes and, more specifically, relates to evacuation systems for scanning electron microscopes.
2. Prior Art
Electron microscopes, including scanning electron microscopes, require a relatively high vacuum in order to operate for a variety of reasons.
The electron beam sources used to produce the beam cannot operate properly unless maintained in a relatively high vacuum. Atmospheric gas molecules within the gun assembly will be ionized by the passage of the electron beam and the liberated electrons and ions will flow to the anode and cathode, respectively. This may result in current instabilities as well as potentially severe and damaging arcing. Even at low concentrations, the acceleration of ions into the cathode results in a shortened operational life for the gun assembly. Consequently, a relatively high vacuum is required within a gun assembly to protect the emitter.
The electron beam can be scattered by the presence of molecules within the beam path. Consequently, to maintain a highly focused beam, such scattering must be minimized by maintaining a relatively high vacuum within the beam tube.
The presence of volatile contaminants within the optical system needs to be avoided since these contaminants may be polymerized by the impingement of the beam. Polymerization of the contaminants can form a coating upon surfaces which may alter the optical properties of the electron microscope. Likewise, impingement of the beam upon the specimen may cause volatile organics to be polymerized and to form a coating on the specimen surface. This layer of contamination can quickly obscure small details. These additional possibilities require a clean vacuum to prevent system or sample contamination.
Vacuum systems for evacuating scanning electron microscopes have been used for a number of years. The most basic configuration of a scanning electron microscope vacuum system provides an oil diffusion pump mounted directly beneath the specimen chamber. The evacuation of the gun is accomplished via the beam tube. The arrangement is very simple but does not result in effective evacuation of the gun assembly due to the small channel (i.e., the beam tube) through which it must be evacuated.
An alternative arrangement was designed in which a small bypass tube is provided to connect the gun assembly to the specimen chamber. This arrangement offers some advantages over previous models but still is limited by the fact that all of the evacuation is being performed through the specimen chamber. Consequently, the vacuum which is obtainable in the gun assembly can be no better than that present in the specimen chamber. This can be particularly troublesome when dealing with specimens which "outgas".
A vacuum system for a scanning electron microscope was designed which somewhat decouples the evacuation of the gun cavity from that of the specimen chamber, as shown in FIG. 1. The vacuum pump 10 is connected to a manifold duct 12, or cavity, separate from the specimen chamber 14. The connections to the specimen chamber 14 and the gun assembly 16 proceed from the manifold duct 12. This arrangement is commonly used in more expensive scanning electron microscopes in one form or another. This arrangement still suffers from the disadvantage of having a bypass connecting pipe 18 of relatively small diameter extending between the manifold duct 12 and the gun assembly 16. This arrangement effectively limits the degree of vacuum which can be achieved in the gun assembly 16.
The oil diffusion pump has been commonly used as the means for evacuating a scanning electron microscope since it historically offered the highest pumping speed for the lowest price. The principle of operation of the oil diffusion pump is the entrapment of gas molecules in a supersonic jet of oil vapor. The disadvantage of oil diffusion pumps is that the disruption of the jet, such as through an inrush of air, will cause turbulence in the jet stream and the pumping oil will then be dispersed throughout the system being pumped. Consequently, other types of high vacuum pumps are increasingly being used in applications where oil contamination is unacceptable.
Scanning electron microscopes have increasingly been equipped with turbomolecular pumps known as turbo pumps. The turbo pump employs several stages of high-speed turbine blades. Gas molecules striking the spinning blades are given momentum which impels them further into the successive compression stages of the pump where they are ultimately removed. The principal advantage of the turbo pump is that it is intrinsically oil free. However, in order to use a turbo pump in a scanning electron microscope, it is necessary to deal with the high-frequency vibrations produced by such pumps. Though turbo pumps have become increasingly quiet, they still produce an unacceptable level of vibration by scanning electron microscope standards. Consequently, it is necessary to provide a system for mechanically decoupling the turbo pump from the optics of the scanning electron microscope. Traditionally, this has been done by suspending the pump 10 from a metal bellows 20, as shown in FIG. 2. An elastomer sleeve 22 surrounding the metal bellows 20 has also been utilized. The suspension by metal bellows 20 represents an expensive and cumbersome arrangement. The use of metal bellows suffers from several disadvantages. First, the use of metal bellows is an expensive isolation process. Second, the flexibility of the metal bellows limits the orientation of the pump being utilized. Third, the metal bellows have a large internal surface area with numerous small angles which can act to trap gases minimizing the effectiveness of the evacuation system. Additionally, unless the metal bellows are constrained in some fashion, the suspended turbo pump may move a considerable distance when the system is pumped or vented. The repeated flexing may result in metal fatigue and consequent failure of the metal bellows. Furthermore, metal-to-metal conductive paths through the bellows may transmit unwanted high-frequency vibrations.
Another isolation solution has been proposed using a "fat O-ring" 24 positioned so as to be axially compressed between the turbo pump 10 and the system to be isolated, shown in FIG. 3. The axial compression O-ring 24 is intended to act as a compliant isolator between the pump and the system. Implementation of the axial compression O-ring 24 requires an additional coupling device (not shown) to maintain tension across the axial compression O-ring 24 during nonuse. Without such additional coupling devices, the system would disassociate when not evacuated. Consequently, effective isolation must also be achieved through the additional coupling mechanism. The resulting isolation system represents a complex arrangement with no significant advantages over the metal bellows isolation system.
The object of the present invention is to provide a turbo-pumped scanning electron microscope which overcomes the aforementioned drawbacks of the prior art. Additionally, the object of the present invention is to provide a configuration for a vacuum system which results in an improved pumping of the vital gun area for scanning electron microscopes. A further object of the present invention is to provide an improved vibration isolation device which is compact, inexpensive, highly effective and which permits mounting of a turbo pump in a horizontal orientation substantially perpendicular to the beam tube.